The ability for ballistic helmets to stop rifle rounds is a relatively new thing, kevlar helmets will only stop a small percentage of rifle rounds at certain ranges.
The purpose of this video is to dispel myths that surround what bullets do when they hit helmets directly. It also serves to illustrate the incredible pressure that is produced buy a high velocity projectile.
Keep in mind that helmets are only rated to the level IIIA which only reliably stops pistol rounds up to 1,550 FPS. So the question comes up, "why wear helmets at all?" The answer is that shrapnel and blunt trauma kill far more people than direct bullet strikes. A flying piece of asphalt from a road side bomb kills just as sure as a bullet, and modern ballistic helmets do an exceptional job of protecting the wearer from those kind of threats.